Question
my plant (a purple moth orchid) has a white powder (mould? Fungus?) substance on the
underside of its bottom-most leaves...what is it? The two top leaves are fine, they're green
and healthy-looking, as are the ones on the layer beneath that, but the leaves on the bottom
aren't looking too good (this is probably a stupid question, but I'm a newbie, so, do the leaves
on the bottom die naturally? I've been watering etc, so I'm not sure what would cause this...)
Thanks!

Answer
Hello Kess,

While I cannot be certain without actually seeing it, I do not think it is a mold or fungus but believe the white substance might be the insects mealybugs or scale.  Mealybugs look like tiny cotton balls while scale look like tiny turtle shells.  Which ever one it is, they are bad news for your orchid plant because they will slowly kill it by sucking the life out of the plant.  Try wetting a paper towel with isopropyl alcohol and wiping the underside of the leaf, if you find tiny black or brown insects on it, they probably are mealybugs.  Scale are a little harder to take off as they attach their shell onto the leaf.  

To get rid of either or both will require repotting plus treating the plants with a systemic insecticide as wiping the leaf only temporarily gets rid of them because there are many more down within the pot waiting to come up.  During repotting, clean the leaves of any insects that you can see.  After repotting, go to your local garden center or nursery and ask them for a systemic insecticide recommended for orchids, mix it according to the directions then soak the pot in it for about 30 minutes.  You will have to do this 3 times at 7 to 10 day intervals to make sure you've killed them off.  

Lastly, yes it is normal for the bottom leaf to die and fall off because they usually are the oldest leaf on the plant.  What is not normal is for upper leafs to die before the bottom leaf does.  Also, it is not unusual for one or two leafs to die-off during the year while the plant grows one, two or even three new leafs to replace them.  

Hope this helped.

Jim Kawasaki
San Jose, Ca.